Currently Bi-stable Reeled Composite (BRC) material technology as described in WO 97/35706 entitled “an extendable member”, and WO 88/08620 entitled “an elongated hollow element”, is used in a screen product, as per WO 2010/109247 entitled “screens”. The BRC technology allows a rigid hollow Split Tube Extendable Member (STEM) to roll up on itself to change from a rolled up state (1) to an extended state (2). This is shown in FIG. 1.
As used herein the term “Split Tube Extendable Member (STEM)” or “split tube” or simply “tube” means a bi-stable member that has a first stable state (rolled up state) in which it can be rolled up flat in a coil and a second stable state (extended state) in which it is in the shape of a tube which forms as it is extended from the rolled up state. Usually, in the extended state, the tube is self-supporting and also capable of supporting other materials. STEMs can be used to form screens, for example to provide privacy around a hospital bed, as shown in FIG. 2; the screens may be made of a framework having STEMs forming the top strut (and usually a bottom strut as well) of the framework, with the uprights of the framework supporting the STEMs. A screen can be held within this framework, as will be described in more detail later. One substantial advantage of such an arrangement, as described in WO2010/109247 and used in the present invention, is that the STEMs can be rolled up, usually together with the screen itself, thereby retracting the screen. The screens are therefore made of rollable material and often of a material that is substantially inelastic to provide a taut, stiff screen. This is the arrangement used in the present invention, which provides improvements on the arrangement described in WO2010/109247.
An example of the arrangement described in WO2010/109247 will now be given by reference to FIG. 3. The mechanical properties of a frame structure employing split tubes (5) are dependent upon the stiffness of the split tubes. Such a structure comprises split tubes as its upper and lower edges, with a body (41) and handle (40) as the vertical supporting sides. The split tubes (5) are rolled up inside the body (41) at one end, while the other end is attached to a handle (40). The split tubes can be pulled out from the body into the extended position shown in FIGS. 2 and 3 by pulling on the handle (40), which unrolls the split tubes and causes the tubes to be pulled out from the body (41) to change from the rolled up (flat) configuration (FIG. 1) to the extended (tubular) configuration thereby deploying the screen.
When the screen has been deployed, as shown in FIGS. 2 and 3, the load exerted from attaching a screen panel (7) between the top and bottom split tubes (5) as well as gravity exerted on the tubes themselves causes the split tubes to experience a vertical displacement V (6) at the midpoint (8) of their span, as shown in FIG. 3. It is desirable for this vertical force exerted on the split tubes (5) by the screen panel (7) to be evenly distributed along the span of the split tube, in order to prevent twisting and/or buckling of the split tubes. The present invention details methods by which the effects of this twisting and/or buckling phenomena can be lessened or eradicated.
Upon the transition between a rolled up state (1) to an extended state (2), a split tube (5) undergoes a complex physical geometric transformation, as shown in FIG. 18. Two properties of note are:                the change in linear width, W of the split tube between the two states: the linear width of the extended tube, We (34) is less than the linear width of the tube in the closed rolled up state, Wc (33); and        the natural length L (35) over which this transition occurs.        
It is these parameters which define the natural edge profile of a split tube, as shown in FIG. 18. The transition region between the flat state (1) of a split tube and its tubular extended state means that the inner region of a frame structure employing split tubes is not strictly rectangular. The product described in WO 2010/109247, as shown in FIG. 2, has the longitudinal edge of the screen panel (22) attaching to the longitudinal edge of the split tube (5) via a zip. The end of the split tubes (5) at the centre of the coil (1) shown in FIG. 1 is secured to an axle within the body (41) via means of a flat clamp, forcing the tube into the flat, coiled state (1), as shown in FIGS. 1 and 19. The tube (5) is clamped at its free end to the handle (40) in its tubular state, which urges the tube to adopt a tubular configuration (2). This poses a technical problem at the transition of the split tube's two states.
Currently the split tube (5) technology has many applications and is generally used either as a single tube or as part of a larger structure, with elements connecting to one or more tubes. A retractable room divider as described in WO 2010/109247 (FIG. 2) is an example of a larger structure design using split tubes (5). This divider uses two parallel horizontal split tubes (5) as the top and bottom edges of the frame structure, to which a screen panel is attached. Such a screen panel, which is indicated by the reference number 22 in FIG. 11, is attached to the two split tubes (5) via means of a zip (10), whereby for each of the two horizontal edges of the screen, one half of a zip is sewn to the screen, and the other half to a split tube. The transition between the flat and tubular states of the split tubes at the corners of the screen (22) causes the screen to pucker at these corners.